An electro-optic polarization converter is important for a variety of applications. For example, in a single-mode optical fiber system, it may serve as a polarization-controlling device for typical single mode fiber transmission systems in which the propagating waves normally do not preserve their initial linear polarization so that periodically restoration to a linearly polarized state of the waves becomes important.
In the January 1986 issue of Optics Letters, Vol. II, No. 1, pps. 39-41, there is described a TE-TM mode converter which employes an X-cut lithium niobate (LiNbO.sub.3) crystal substrate in which there is provided a titanium in-diffused channel waveguide which supports one TE and one TM mode. The waveguide propagation direction is along the optic Z-axis of the crystal. The electro-optically induced linear polarization rotation is accomplished through the off-diagonal r.sub.61 electro optic coefficient. The electrode structure for applying the electronic field for inducing the polarization conversion consists of three separate electrodes symmetrically located with respect to the waveguide and including a center electrode overlying the waveguide and a pair of outer electrodes, one on each side of the center electrode. One of the outer electrodes is grounded, and two independent voltages V.sub.1 and V.sub.2 are applied to the center electrode and other outer electrode, respectively. Voltage V.sub.1 contributes a mainly vertical electric field component and V.sub.2 a mainly horizontal field component inside the waveguide. By varying the V.sub.1 and V.sub.2 appropriately, conversion between the TE and TM modes of a single mode optical wave applied to one end of the waveguide is effected.
In this arrangement, the vertical electric field provided by V.sub.1 provides the electro-optically induced TE-TM mode coupling through the off diagonal r.sub.61 electro optic coefficient, and the electric field provided by the voltage V.sub.2 is used to provide the necessary electro-optically induced phase shifts between the TE and TM modes through the complementary r.sub.22 and r.sub.12 electro optic coefficients. Since the wavelength propagation direction is along the optic axis, both TE and TM modes see the same ordinary index and are therefore already nearly phase-matched.
This device has several drawbacks. Firstly, optical isolation needs to be provided between the central electrode and the underlying waveguide. This typically involves use of a buffer layer between the LiNbO.sub.3 substrate and the overlying electrodes which makes for processing complexity.
Additionally, the performance is extremely sensitive to the electrode alignment. Accurately aligning the three electrodes, typically separated by a few microns, makes for manufacturing complexity.
It is desirable to provide a mode converter having a less complex structure than the structure discussed herein above.